In order to drive a light emitting element (such as organic EL, light emitting diodes etc.) controlled by an electric current, that is, to drive an electric current element, accurate control of the electric current to be supplied to the electric current element is required in a range from minute electric currents for low gray scales to large electric currents for high gray scales. If a conventional simple matrix drive is employed to an organic EL display device, high luminance drive is required especially in a high gray scale region due to a low duty ratio, thereby shortening a life of the organic EL display device. For this reason, an active matrix drive using TFT is mainly employed.
By utilizing a signal programmed in a selection period, the active matrix drive makes it possible to perform the driving in a hold mode in which light is emitted also during a non-selection period other than the selection period.
Recently, organic EL elements have been improved to be more efficient, thereby requiring that more minute electric current should be controlled more accurately at a higher speed. Various driving methods have been proposed, but none of them is a breakthrough solution. Thus, it is expected that a demand for a driving technique for coping with finer resolutions and gray scale increases will be higher.
FIG. 9 is a circuit diagram illustrating a conventional driving circuit described in Patent Literature 1. In the driving circuit illustrated in FIG. 9, a gate electrode of a transistor 10 is connected with a scanning line Xi, a drain electrode of the transistor 10 is connected with a drain electrode of a transistor 12. The drain electrode of the transistor 12 is connected with a power source line Vi. A gate electrode of the transistor 12 is connected with a source electrode of the transistor 10. A source electrode of the transistor 12 is connected with a drain electrode of a transistor 11 and with anodes of organic EL elements Ei and Ej. A gate electrode of the transistor 11 is connected with the scanning line Xi, and a source electrode of the transistor 11 is connected with a signal line Yj.
During a selection period, a power source signal voltage is applied on the power source line Vi. The power source signal voltage is equal to or lower than a reference potential Vss. When the scanning line Xi becomes H (high) during the selection period, the transistors 10 to 12 are turned on. Meanwhile, a voltage across each organic EL element Ei and EJ becomes 0 or reversely biased. Thus, a programmed sink current Ij flows in a path indicated by the arrow α.
When the transistor 12 is turned on in the selection period, a gate-source voltage Vgs determined according to a driving capacity of the transistor 12 is applied on a capacitor 13. By this, an electric charge corresponding to the gate-source voltage Vgs is stored in the capacitor 13.
After that, in a non-selection period in which the scanning line Xi becomes L (low) after the selection period is ended, the capacitor 13 thus charged during the selection period applies a positive voltage across the gate and source of the transistor 12, thereby turning on only the transistor 12.
Moreover, a power source signal voltage to be applied on the power source line Vi during the non-selection period is a power source voltage Vdd that is sufficiently higher than the reference potential Vss. Thus, a forwardly biased voltage is applied on the organic EL elements Ei and Ej. The transistor 12 supplies the organic EL elements with a constant electric current whose ampere is equal to Ij. That is, it is possible to supply a constant electric current to the organic EL elements Ei and Ej even if the transistors 12 are uneven in terms of properties.